Using larger sd disck will let you put a lot of stuff on a card. But on the down side, the more you put the longer it takes to get to a free spot on the disk. Indexing being what it is.
Optimum performance is 4 to 8 gb and then dont keep stuff on the disk. record it and then transfer it to the computer especially if you are recording sonar data.
Waypoints should be the exception as you will use them more often.
Chuck